PROJECT SUMMARY There is a critical need to develop treatments for muscle wasting, which increases morbidity and mortality of patients suffering from muscular dystrophies, myopathies, cancer, kidney failure and pulmonary disease. Stem cell therapies require the ability to generate muscles of particular sizes and shapes to replace damaged muscles in, for example, the round eye or the elongated limb. A thorough understanding of how muscles with specific properties develop will allow us to program muscle cells to adopt specific properties, a key step towards developing these kinds of treatments. The long term goal of the proposed work is to use the fruit fly, Drosophila melanogaster, to determine the cellular mechanisms underlying the development of somatic muscles with distinct sizes, shapes, orientations, innervations, attachments and gene expression patterns. Within each muscle these properties are encoded by distinct sets of gene regulatory factors, including the conserved genes Apterous (Ap), Midline (Mid), and Muscle-segment homebox (Msh). We are focused on a subset of embryonic abdominal muscles that express Ap, Mid and Msh. We have found that expression of Ap outside its normal pattern dramatically disrupts the musculature, leading to changes in muscle positioning and loss of muscle attachments. The failure of the muscle-tendon connection prevents these mutant embryos from generating the force needed to hatch from its eggshell and leads to death. Our hypothesis is that Ap directly regulates genes involved in muscle orientation and the selection of direct versus indirect muscle attachment by functioning in a network of gene regulators including Mid and Msh. We will test this hypothesis with three Aims: 1) examining the location and level of muscle guidance and attachment factors in Ap mutant backgrounds; 2) identifying the genes regulated by Ap; and 3) determining the interactions between Ap, Msh and Mid during muscle development. The proposed work combines gene expression analyses with cell biology, microscopy and genetics to learn how muscle connections are made and determine how many genes are targets of Ap regulation. Taken together, the experiments described in this proposal will determine how muscle properties like shape, orientation and attachment type are specified, which will inform our understanding of the vertebrate orthologs of muscle development in humans and lead to the development of therapies.